Which of the following challenges relates to targets in less than ideal indirect fire illumination?

The challenge related to targets in less than ideal indirect fire illumination primarily involves complex detection environments. In such environments, various factors may hinder the ability to accurately identify and engage targets. These could include terrain features that obscure visibility, varying light conditions that affect perception, and the presence of distractions that complicate target recognition. Indirect fire often relies heavily on the observer's ability to detect and accurately report target positions, and complex environments can significantly impede this crucial process.

While the other options may present individual challenges, they do not encompass the broad set of difficulties posed by diverse and complicated conditions that influence visibility and target identification. For instance, behavioral deficiencies of the observer focus more on the individual’s capability rather than the external conditions affecting detection. Similarly, camouflaged targets and small targets in bright conditions are specific scenarios, whereas complex detection environments capture a wider range of challenges faced in indirect fire scenarios.